This invention relates to mixing circuits in general and more particularly to a mixer circuit where the effects of local oscillator leakage is reduced and particularly adapted to be utilized in a zero intermediate frequency (IF) design.
Zero IF type receivers are well known in the prior art and essentially the zero IF type receivers skip the step of going to IF frequency and instead converts the desired incoming signal directly to baseband in a single operation. In addition to the economy of direct conversion there are performance advantages particularly for multiple-function radio receivers. To date, the zero IF type receiver has found only limited use primarily due to the problems resulting from offsets present at the output of mixer circuits which are caused by imperfection in performance.
In particular a very troublesome problem in regard to such mixer circuits is due to mixer leakage. In any event, as indicated above, the zero IF approach has been widely described in the prior art. U.S. Pat. No. 4,328,850 issued on Dec. 9, 1980 to I. A. W. Vance and entitled TRANSMITTER/RECEIVER FOR SINGLE CHANNEL DUPLEX COMMUNICATION SYSTEM describes a transmitter/receiver which can be used for a cordless telephone. The receiver produces direct conversion from the radio frequency. The local oscillator frequency is modulated by an audio signal to be sent, the modulated signal being passed through a dual splitting combining network.
As described in the patent, the receiver makes use of the zero IF or direct conversion method of demodulation. Such a method is also described in British application specification Ser. No. 1,530,602 published Nov. 1, 1978 by I. A. W. Vance.
U.S. Pat. No. 4,470,147 entitled RADIO RECEIVER WITH QUADRATURE DEMODULATION AND DIGITAL PROCESSING issued on Sept. 4, 1984 to J. K. Goatcher and is assigned to the International Standard Electric Corp. This patent depicts a radio receiver which divides the radio frequency input into two channels and in each channel mixes it with the carrier wave frequency. A quadrature shift in a mixed carrier wave frequency enables baseband signals to be filtered from the mixer outputs. Essentially, the analog quadrature signals are converted to digital form and processed digitally to produce the original modulating signal for AM/FM or PM transmissions.
The single sideband modulation and mixing frequency is the sideband center frequency instead of the carrier frequency.
U.S. Pat. No. 4,476,585 entitled baseband demodulator for FM signals issued on Oct. 9, 1984 to J. Reed and is assigned to the International Telephone and Telegraph Corp. shows a modulator which is employed in the zero IF system and uses a local oscillator for providing quadrature output signals at the center frequency of the FM signal to be demodulated. A demodulator has first and second mixers for separately mixing the FM signal with the quadrature signal to provide a first and second output signal, each in quadrature at the outputs of the mixers.
These signals are lowpass filtered. A demodulator is shown which includes third and fourth mixers with each mixer receiving at an input the output of one lowpass filter. At another input the mixers receive the third and fourth signals. The third and fourth signals are derived from mixing a variable controlled oscillator signal with a local oscillator quadrature signal. Essentially, the patent also shows the switching circuit which operates to alternate the third and fourth signals as applied to the input of the third and fourth means as well as the output as applied to the difference amplifier and uses an additional amplifier coupled to the outputs of the mixers so that one can utilize automatic gain control in a zero IF system.
As indicated above, the prior art is fully cognizant of the zero IF technique and it is further evidenced by other U.S. Pat. Nos. which are as follows: 4,540,958, 4,476,585, 4,480,327, 4,462,107, 4,488,064, 4,506,262, 4,521,892, 4,525,835, 4,322,851 and 4,254,503.
The main purpose of such a system is to provide a communication system where the amount of tuned circuitry employed is substantially reduced. In obtaining a reduction in the number of tuned circuits, one is therefore able to integrate large portions of the receiver and produce radio receivers which are extremely compact and reliable. These receivers, as indicated may be employed in many areas such as in selective paging systems and so on. Thus to achieve such advantages, the design of such receivers is implemented according to the zero IF technique. As indicated, in such a system there is present a receiver in which the local oscillator signals are in phase quadrature at the carrier frequency and are each separately mixed with the incoming audio modulated signal. The resultant signals have a zero IF with the two sidebands folded over on each other at the baseband extending in frequency from DC to the single sideband wave of the original signal.
As one can see from the above-cited references, the concentration has mainly been in regard to implementing receiver design or a transmitter design utilizing baseband circuitry. As indicated, baseband circuitry operates at frequencies where integrated circuit technology is well established and therefore a transceiver which consists of primarily of integrated circuits is relatively small and inexpensive.
It is of course a desire of the prior art to obtain improved performance from the receiver. A major difficulty which arises with conventional zero IF receiver architecture revolves around the DC offsets which are present at the output of the mixers. These offsets on the order of millivolts in the presence of generally much smaller desired signals causes the low-noise baseband amplifiers to saturate before the desired signals are amplified efficiently to interface with the analog to digital converter.
In the zero IF receiver, there is a particular problem which results because of the fact that the input frequency is the same as the local oscillator frequency. As one can understand, mixing is a process of multiplication whereby the local oscillator input is multiplied by the input signal or input frequency. When the two frequencies are equal, as in the case of a zero IF receiver, a DC term results. In any event, mixers are not perfect circuits and typically a mixer exhibits a leakage from the local oscillator input port into the signal input port or input section.
Since the local oscillator frequency is the same as the input frequency, this leakage is in fact at the same frequency as the local oscillator frequency. Therefore, a DC term results purely as a result of this leakage. This DC term appears as a genuine input which is applied to the input terminal. Thus as one can ascertain, this results in an undesirable DC voltage at the output of the mixer which essentially appears as an interfering signal. Hence it is an object of the present invention to provide an improved mixer apparatus which substantially reduces the effects of local oscillator leakage.
It is a further object of the present invention to provide a cancellation technique for reducing mixer leakage particularly in a zero IF receiver.